It has become increasingly evident that hemodynamic shear stresses play a important role in the normal physiology and pathobiology of the vascular wall. Some of the physiological roles of blood-induced shear stresses are flow-dependent dilation and flow-dependent growth of vessel diameter. Blood flow dynamics have also been implicated in such disease processes such as atherogenesis and hypertension. In vitro studies have also shown that shear stress can alter several cell functions, such as prostacyclin production, histamine forming capacity, angiotensin converting enzyme activity release, fluid-phase endocytosis, and cytoskeletal structure. Despite the numerous examples of the effect of shear stress on the endothelium, the mechanism of action of shear stress has yet to be determined. The point of action of these stresses is the interface between the vascular wall and the blood; i. e. the endothelial membrane. We therefore hypothesize that the application of shear stress on the endothelial membrane leads to increased levels of intracellular second messengers, which then trigger a cellular response. Specifically, we hypothesize that shear stress causes membrane perturbations which lead to the hydrolysis of the polyphosphoinositides and increased intracellular free calcium. The resultant primary second messengers activate cellular processes directly or indirectly by activating specific protein kinases. The aim of the proposed research is to test this hypothesis by measuring polyphosphoinositide turnover and intracellular calcium. Using shear stress-induced prostacyclin production and angiotensin converting enzyme activity release as key markers of endothelial membrane function, we will attempt to elucidate the signalling pathway. Such an investigation will allow one to predict which other endothelial cell properties are affected by shear stress, and to what degree. It will increase the understanding of the importance of hemodynamic shear in physiological processes and shed light on how shear stress can be involved in the propagation of disease processes.